The present invention relates to a method and apparatus for forming a directioned and controlled suspension spray of a pulverous material and a reaction gas.
There are numerous descriptions in the literature of the feeding of a suspension into a reaction chamber. Most of them deal with either direct blowing of a pneumatically carried finely-divided solid material or apparatus in which the suspension spray is formed as if in an ejector by means of pressure pulses produced in the reaction gas, and is blown into the reaction chamber. Such a spray forms a cone in which the concentration of solid is highest in the center of the spray. The shape of the distribution is primarily dependent on the properties of the solid and the flow velocity of the suspension. The solid and the gas have in this case substantially the same direction.
As known, the transfer of mass between a reacting solid particle and the gas surrounding it is substantially dependent on the difference in velocity between them. For this reason it is important that the velocity difference is greatest or maximal in the reaction chamber itself. For this reason, mixing the reaction gas and the pulverous material in the reaction chamber itself is a more advantageous method of forming the suspension than is the ejector-like method described in the above-mentioned example. When the gas and the solid material are mixed in the reaction chamber, the velocity difference is at its greatest when the solid particles have not yet had the time to settle at the velocity of the gas flow.
As an example of such suspension-forming in the reaction shaft can be mentioned FI Pat. No. 57 786, in which the procedure is briefly as follows: An annular, downwards directed flow of solid is formed of a pulverous material by means of partial flows falling on an inclined surface. The reaction gas, which has been brought into a strong turbulent motion in a specific turbulence chamber, is allowed to discharge, parallel to its axis of rotation, via a throttling evening-out member at the end of the turbulence chamber, to inside the annular flow of the pulverous material. From this outlet, which opens directly into the reaction chamber, the strongly turbulent spray discharges as a cone the flare angle of which can be adjusted within a range of 15.degree.-180.degree., and the spray meets, at the necessary velocity difference, the flow of pulverous material in the reaction chamber itself.
For effective and economical utilization of the reaction chamber, it is necessary that the suspension spray, whether formed before entering the reaction chamber or in the reaction chamber itself, has a controlled direction and spread.
For reactions it is important that the mixing ratio of reaction gas to pulverous material is correct at every point of the reaction chamber. Regarding the use of the space, it is also advantageous that the directioning and distribution device be as small as possible, and that the reaction chamber be as well filled as possible; however, in this case it is necessary to take into consideration the either wearing or thickening effect of the suspension spray on the walls. This leads to a need of controlled directioning of the suspension spray of the reaction gas and the pulverous material, usually symmetrically in relation to the reaction chamber, in spite of the difficulty due to the fact that the gases are often introduced into the reaction chamber at an angle awkward in relation to the main flow. One known method of directioning gas sprays is to make use of a strong rotary motion, as in the above-mentioned FI Pat. No. 57 786. This is often even necessary in processes which require very demanding reaction conditions. In this case, however, it is necessary to use a certain amount of pressure energy, in which there can often be achieved savings in less demanding processes.
Perhaps the simplest method of deflecting a gas flow, which often arrives in an almost horizontal direction, to a direction parallel to a vertical reaction chamber is to use an elbow pipe of uniform thickness. This has the advantage of simplicity and a rather small pressure loss, but also the disadvantage of an asymmetrical discharge gas flow. From the literature (Handbook of Fluid Dynamics, Victor L. Streeter, McGraw-Hill Book Company, Inc. 1961, pp. 3-18 . . . 3-23, 9-11, 14-16.) it is also known to change a pipe flow at the elbow of a pipe as the centrifugal forces have a stronger effect in the center of the pipe than on the sides, owing to the difference in the radii, thereby causing a concentration of the flow in the central area of the pipe towards the outer wall and thereby effects the formation of two vortices deflecting from the walls of the pipe. For suspension formation there has to be added inside the pipe elbow a tubular member for feeding pulverous material, and this member increases the rate of the one-sided gas discharge flow produced by the above-mentioned phenomenon, as well as the pressure loss.
Even a higher model than this is the sufficiently long (length/diameter is great) straight pipe commonly used for the directioning. In spite of its simplicity this pipe is usually too long (high) in metallurgical processes, and it is difficult to install inside it a replaceable feeding device for pulverous material. One conventional solution for deflecting and directioning of gas is to direct it via a relatively large chamber into the reaction chamber by throttling it sufficiently before discharge into the reaction chamber. For practical reasons (excessive discharge velocity or respectively too large a distribution chamber, and in both cases too great a pressure loss) it is often impossible to bring the throttling to a sufficient degree, in which case the directioning is not successful. Instead of one discharge outlet it is, of course, possible to use several of them (a grating), in which case higher discharge velocities can be used, D. R. Richardson in his article "How to design fluid-flow distributors", Chem. Eng., 68 No. 9, 83-86 (1961) has determined, for the grating, the value of pressure loss necessary for ensuring the evenness of the discharge rate prevailing in the openings of the grating, or of the distribution of the gas amount over the grating, the value having to be at least 100-fold compared with the pressure loss based on the inlet velocity. Even in this case the evenness and directioning can be achieved, but at the expense of a large size and a pressure loss.
Very good directioning is provided by a method of introducing the reaction gas into the gas distribution chamber located on the central axis of the reaction chamber, from three or several directions symmetrically and by allowing the thus symmetrically formed annular gas flow to discharge into the reaction chamber, and by feeding the pulverous material centrally from inside of the annular gas flow. This is advantageous and even recommendable when what is involved is the reduce into one the unit for forming a suspension of, for example, three or several reaction gases and a pulverous material, since in that case there are available already the gas distribution pipes to be connected to the distribution chamber. If, however, the question is of constructing a completely new unit, it is not worth making the above-mentioned distribution pipes owing to the great losses of material and heat; instead, it is advantageous to use the low single-channel gas-feeding unit with a small pressure loss according to the present invention.
Since in metallurgical smelting apparatus, especially in suspension smelting, there is required a device or devices by means of which the reaction gas and a pulverous material are fed into the reaction chamber to produce a good mixing with each other, it has been necessary to pay special attention to the method of forming the suspension, especially as the size of the smelting units is on the increase.
Two principles are used for feeding a suspension of a reaction gas and a pulverous material into a reaction chamber, and according to these principles the suspension is formed either prior to the actual blow-feeding device or by means of the blow-feeding device itself.
The former method is used in the conventional carbon burners of carbon dust heating or in metallurgical apparatus in which a pneumatically carried finely-divided ore or concentrate is blown, together with its carrier gas, directly into the reaction vessel. When this method is applied, the blow-feed velocity must be adjusted to such a rate that no blow-back of the reactions can occur. When high degrees of preheating are used or in other cases in which the suspension to be formed is very reactive, as in oxygen smelting of a metallurgical sulfide concentrate, the suspension must be formed as close as possible to the reaction chamber or, ideally, in the reaction chamber itself as according to the present invention.
The object of the present invention is to provide a method for forming a suspension, a method in which the first contact between the reacting materials occurs in the reaction chamber itself, so that it is also applicable to forming a suspension of highly reactive materials.